I. Field to Which Invention Relates
The invention relates to a device for limiting the pressing force to a pre-established value in mechanical presses with an upper punch, which is mounted on the upper tup via a hydraulic cushion, which is connected via an overpressure or pressure-relief valve and a pump with a pressure medium supply held in a pressureless condition.
II. The Prior Art
Such mechanical presses are used, usually in the form of thrust crank, eccentric or toggle lever presses for example for the compaction of pulverulent compositions to form solid bodies.
In the compaction such powder or pulverulent compositions to form solid bodies a basic distinction is drawn between two types of methods.
In the case of pressing to a constant height, which is used in the majority of cases, it is desired to produce bodies keeping to fixed dimensions. Since the properties of the material to be pressed are generally very constant, if the pressing die is filled to the proper volume only slight changes or variations in density occur between one pressed component and another.
In pressing to a constant pressure, the aim is that of pressing every pressed component with the same pressing or compression force so that, even in the case of different degrees of filling, components or parts with the same density are produced. This requirement can, however, only be achieved with the sacrifice of different heights of the components or parts, that is to say, the upper punch must be able to vary correspondingly in position, that is, compensation for the difference in component thickness.
In the case of material to be pressed with difficult filling properties and varying bulk density, the requirement more particularly comes into existence of undertaking the pressing operation with a constant pressure parameter. Since in this respect mouldings or pressings with a different height are produced, the pressing must generally be subsequently mehcanically worked or machined in order to achieve the correct final dimensions. This method is used for example in the case of the production of UO.sub.2 pellets for reactor fuel rods and in the case of some types of metallic carbides for wear-resistant parts.
A special case requiring a constant parameter of compression pressure in constituted by the calibration of flat sintered iron parts. The sintered parts often have variations in height of a few tenths of a millimeter. Since in accordance with the part height and the material during subsequent pressing to achieve a constant height, the differences in the part heights are taken up by the elasticity of the tool and the machine, pressing to a constant height is, in this case, liable to lead to excessively high loading of the tool and of the machine. Therefore, in such cases pressing to a constant pressure is preferred and the resulting pressings are calibrated or manufactured with the same pressing force, though in some circumstances they may have different heights.
In the case of pressing to a constant height, small, harmless variations in pressure must be compensated for without excessively high variations in the part height of the resulting pressings. If, however, owing to incorrect functioning or improper operation, for example, when a second filling is placed in the die without ejection of the previously produced pressing, danger of overloading of the tool or the press results and the device should be capable of avoiding such overloading or damage, that is to say, the upper punch must be able to yield without further buildup of pressure and the machine must be stopped.
In the case of pressing to the same density, the device should be capable of compensating for differences in the density of filling, that is to say, the pressing force should always achieve the same predetermined pressure. After achieving this predetermined pressure, the upper punch must be able to yield in an upward direction without a further increase in pressure. At the same time, however, the device must also ensure that, in the case of faulty operation, no overload occurs.
This problem has been recognized and various attempts have been made to solve it.
Thus, for example, the measure has been adopted of connecting with the hydraulic cushion a pressure accumulator, which is loaded with nitrogen at the respective opening pressure. This system has disadvantages, however. On changing the pressing force, it is necessary to charge the accumulator with additional gas from a nitrogen cylinder or nitrogen must be removed from the pressure accumulator. The gauge or manometor for the pressing force cannot be used to read off or determine the respective working pressure.
In the case of large presses, the hydraulic cushion becomes too large, since nitrogen supply cylinders are generally filled with a maximum pressure of 140 atmospheres gauge.
Finally, the hydraulic cushion must be precharged with the respective pressing force, that is to say, in the nonloaded, i.e., nonactual part-forming condition the pressing plunger exerts on the cover, which is generally attached with screws, a pressure in the order or magnitude of the actual pressing or part-forming force. In the case of large presses, this leads to a very complicated design.
In the case of another previously proposed form of construction, an excess pressure valve is connected between the hydraulic cushion and the pressure accumulator. This construction can, however, not be found satisfactory either.
The valves must be precontrolled or set due to the large quantities of oil flowing through them. Since the valves must respond to the same pressure during each operating stroke, lack of precision in the opening pressure occurs and, therefore, there is a lack of precision as regards the pressing force.
The precise opening pressure can, on the one hand, only be set by spring loading of the excess or overload pressure valve. This in itself also leads to a lack of accuracy.
Since the pressing operation is often carried out with displacement of large volumes of oil, the oil may be considerably heated by flowing through the excess pressure valve. This heating is increased by the fact that it is always the same oil which is displaced backwards and forwards between the hydraulic cushion and the pressure medium accumulator.
In the case of a third prior art device, the preloading of the hydraulic cushion is brought about by its own pumping unit via an excess or overload pressure valve. Certain disadvantages result from this, more particularly, the disadvantage of extra filling of the pressure accumulator, and in the case of large presses, high pressure pumps must be employed. The disadvantages that the respective working pressure cannot be read from the pressing force gauge and the hydraulic cushion must be preloaded at the respective pressing force, nevertheless remain.